Mortgage Loans by Geographic Region (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 1,665.2	$ 1,612.3
Percent of Total	100.00%	100.00%
New England
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	132.4	147.0
Percent of Total	8.00%	9.10%
Mid-Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	161.2	174.1
Percent of Total	9.70%	10.80%
East North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	212.1	212.7
Percent of Total	12.70%	13.20%
West North Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	147.3	151.2
Percent of Total	8.80%	9.40%
South Atlantic
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	418.6	383.8
Percent of Total	25.20%	23.80%
East South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	80.1	52.4
Percent of Total	4.80%	3.30%
West South Central
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	173.1	160.4
Percent of Total	10.40%	9.90%
Mountain
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	81.9	69.5
Percent of Total	4.90%	4.30%
Pacific
Mortgage Loans on Real Estate [Line Items]
Mortgage Loans	$ 258.5	$ 261.2
Percent of Total	15.50%	16.20%